Single-Object Parallax System.
I designed this for the purpose of having a super simple and portable parallax implementation. With this, there's no need for managing so many messy background layers in every room. Just drop the object onto your back-most instance layer and it should handle the rest. Assign a GIF to the object, and let each frame of that GIF be one of your parallax layers. Have as many/little layers as you like, and the object should balance them accordingly.
Though a free example sprite is included, the purpose of this asset is the code. The screenshot examples are the work of an artist who's images I cannot distribute. This is not a background image for sale, this is simply an object that will give depth to your own art or other assets.
Works in GMS2. May need tweaking for GMS1. If you have questions I will try my best to answer them. Enjoy!